Inflation and rupture of vaginal tissue

Abstract

Around 80% of women experience vaginal tears during labour when the diameter of the vagina must increase to allow the passage of a full-term baby. Current techniques for evaluating vaginal tears are qualitative and often lead to an incorrect diagnosis and inadequate treatment, severely compromising the quality of life of women. In order to characterize the failure properties of the vaginal tissue, whole vaginal tracts from rats (n = 18) were subjected to free-extension inflation tests until rupture using a custom-built experimental set-up. The resulting deformations were measured using the digital image correlation technique. Overall, the strain and changes in curvature in the hoop direction were significantly larger relative to the axial direction. At a failure pressure of 110 ± 23 kPa (mean ± s.d.), the hoop and axial stresses were computed to be 970 ± 340 kPa and 490 ± 170 kPa, respectively. Moreover, at such pressure, the hoop and axial strains were found to be $12.8\pm 4.4\text{\%}$ and $6.4\pm 3.7\text{\%}$ , respectively. Rupture of the vaginal specimens always occurred in the hoop direction by tearing along the axial direction. This knowledge about the rupture properties of the vaginal tissue will be crucial for the development of clinical approaches for preventing and mitigating vaginal tearing and the associated short- and long-term traumatic conditions.

Keywords: elasticity; inflation; rupture; vagina.

Figure 1.

Schematic of the vagina mounted onto two needles for free-extension inflation tests. (a) Three-dimensional representation. (b) Two-dimensional representation of the transverse cross section. (Online version in colour.)

Figure 2.

Schematic of the experimental set-up for testing vaginal tissue. A vaginal specimen is mounted to the set up within a water bath and connected to a pressure transducer and computer controlled syringe pump via tubing. Two lights illuminate the specimen surface and two cameras record images of the specimen surface during testing. (Online version in colour.)

Figure 3.

Pressure versus volume data collected from n = 18 vaginal specimens. Data from different specimens are denoted using different colours and labelled with different letters, as denoted by the legend. The rupture point is marked by circular endpoints on each curve. (Online version in colour.)

Figure 4.

Representative strain maps of a vaginal specimen under a pressure of 0 kPa, 62.7 kPa and 141 kPa. (a) Average hoop curvatures were 0.187, 0.165 and 0.146 1/mm and average hoop strains were 0, 0.084 and 0.134. (b) Average axial curvatures were 0.072, 0.065 and 0.058 1/mm and average axial strains were 0, 0.032 and 0.037. (Online version in colour.)

Figure 5.

Pressure versus curvature data in the axial direction (solid line) and hoop direction (dashed line) collected from n = 18 vaginal specimens. Data from the same specimen are reported using the same letter and lines of the same colour, as denoted by the legend. The rupture point is marked by circular endpoints on each curve. (Online version in colour.)

Figure 6.

Pressure versus strain data in the axial direction (solid line) and hoop direction (dashed line) collected from n = 18 vaginal specimens. Data from the same specimen are reported using the same letter and lines of the same colour, as denoted by the legend. The rupture point is marked by circular endpoints on each curve. (Online version in colour.)

Figure 7.

(a) Axial stress versus axial strain data (solid lines) and (b) hoop stress versus hoop strain (dashed lines) collected from n = 18 vaginal specimens. Data from the same specimen are reported using the same letter and lines of the same colour, as denoted by the legend. The rupture point is marked by circular endpoints on each curve. (Online version in colour.)

Figure 8.

Stress data in the axial direction (solid colour, 126±105 kPa) and hoop direction (striped colour, 61.1±19.3 kPa) at 2% strain. This strain value was selected since it was the largest strain achieved by all the tested vaginal specimens (n = 18) in both the hoop and axial directions (*p < 0.05). (Online version in colour.)

Figure 9.

Strain data in the axial direction (solid colour) and hoop direction (striped colour) at three values of the normalized volume collected from n = 18 vaginal specimens. Hoop strains were 0.040±0.008, 0.067±0.016, and 0.094±0.026. Axial strains were 0.023±0.011, 0.036±0.018, and 0.049±0.026 (***p < 0.0005). (Online version in colour.)

Figure 10.

Strain map from a representative vaginal specimen at four pressure values: 80.9, 102, 106 and 105 kPa. Average strain values are also reported above each image. Arrows indicate the location of tearing. (Online version in colour.)